A fuel cell has recently attracted much attention as a cell having a high efficiency and superior environmental characteristics. In general, a fuel cell generates electric energy through a chemical reaction between hydrogen which is a fuel gas and oxygen in the air which is an oxidizing agent gas. As a result of the electrochemical reaction between hydrogen and oxygen, water is produced.
The types of fuel cell include phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, alkaline fuel cells, and polymer electrolyte fuel cells. Of these, a fuel cell system has attracted much attention which uses a polymer electrolyte fuel cell because such a fuel cell system has advantages such as that the system can be started up at normal temperatures and the startup time is short. Such a fuel cell system is used, for example, as a power source of a vehicle, in particular, an electric automobile.
In a fuel cell system, when a stopped state of a fuel cell is continued for a long period of time under a low-temperature environment such as, for example, outdoors in cold climate areas, the valves, pumps, etc. in the fuel cell system may freeze due to moisture such as the product water remaining in the fuel cell system. Because of this, various methods have been developed and proposed for preventing freezing of the valves, pumps, etc. in the fuel cell system.
For example, JP 2004-193102 A discloses a fuel cell operating method in which supply of power from the fuel cell is cut, an external air temperature is detected, a reaction gas is supplied to the reaction gas flow path according to the external air temperature to discharge the water in the reaction gas flow path, and the operation of the fuel cell is stopped.
In a fuel cell system, during operation of the fuel cell, a fuel gas discharged from the fuel cell is compressed by the fuel gas pump such as a hydrogen pump, and the compressed fuel gas is pumped to the fuel cell for reuse. During the operation of the fuel cell, because the fuel gas discharged from the fuel cell contains moisture, water may remain in the valve or the like in the fuel cell system. Because of this, a drying process is applied after the operation of the fuel cell is stopped, by driving the fuel gas pump and pumping the fuel gas.
During the drying process, the moisture remaining in the fuel cell system is absorbed by the fuel cell discharged from the fuel cell. Because of this, the moisture contained in the fuel gas discharged from the fuel cell may condense in the fuel gas pump and condensation may occur. When, for example, the stopped state of the fuel cell is continued under a low-temperature environment, the fuel gas pump may freeze up.